A valve gear for use in vehicle internal combustion engines is provided with a rocker arm for converting a rotary motion of a cam which rotates in conjunction with a crankshaft into a reciprocating motion of a valve stem for a valve, such as, e.g., an intake valve or an exhaust valve. This rocker arm is configured to swing about the pivot axis as a fulcrum located at a longitudinal end portion thereof in accordance with the rotary motion of a cam to cause up-and-down motions of a valve stem by pressing down the swinging end portion of the rocker arm to thereby open and close the valve.
A rocker arm is provided with a valve stem contacting end portion of a U-shape in cross-section at a longitudinal one end thereof so that a valve stem is brought into contact with the bottom wall (connection wall) of the valve stem contacting end portion. Furthermore, at both sides of the lower surface of the connection wall of the valve stem contacting end portion, a pair of valve stem guide walls extending in the longitudinal direction of the rocker arm are provided. It is configured such that a valve stem is disposed between these guide walls to prevent the horizontal displacements of the rocker arm with respect to the valve stem.
This kind of rocker arm is complicated in configuration and required to have high rigidity. Therefore, conventionally, such rocker arm is generally manufactured by forging or precision casting (lost-wax process).
In recent years, however, in accordance with the increased rotating speed and increased output of an internal combustion engine, it has been required to attain a lighter rocker arm to reduce the inertia weight. Thus, a number of techniques for manufacturing a rocker arm by press working have been proposed.
In manufacturing a rocker arm by press working, as a processing method of a valve stem contacting end portion, for example, a processing method as disclosed by the following Patent Document 1 is well known. In this method, a valve stem contacting end portion formed into a U-shape in cross-section by pressing working is subjected to pressure forming to increase the thickness of the lower both sides of the bottom wall of the valve stem contacting end portion to obtain the increased thickness portions, and then the increased thickness portions are subjected to pressure forming to form protruded portions as valve stem guide walls.    Patent Document 1: Japanese Unexamined Laid-open Patent Publication No. 2001-198641 (see claims and FIGS. 4 to 12)